Injury to the anterior cruciate ligament (ACL) is the most common ligament injury in the knee, resulting in approximately 50,000 reconstructions per year in the United States (Frank: C B, Jackson D W, The Science of reconstruction of the anterior cruciate ligament. J Bone Joint Surg. Am 79: 1556, 1997). Unfortunately, approximately 40% of ACL ligaments are improperly misplaced in surgery, which can result in post-operative knee instability, abnormal kinematics, and premature degeneration of structures.
Misplacements in knee surgery are common because ligament reconstruction is a technically challenging procedure for the surgeon. The knee joint complex is interconnected with several structural ligaments and surrounding tissues, which may or may not be functioning normally at the time of surgery.
Trauma related injuries which cause ACL rupture often entail damage to other structures surrounding the knee joint, such as the medial or the lateral collateral ligaments. Additionally, posterior cruciate ligament (PCL) ruptures or tears can occur and require diagnosis and/or surgical reconstruction. These surrounding structures are also critical for insuring adequate knee function, rotatory stability, and normal kinematics.
Although various clinical tests for assessing knee instability exist, it is extremely difficult for the surgeon to objectively take these results into consideration when determining the optimal course of a ligament graft. In particular, the magnitudes and directions of the laxities and their interrelationships to the underlying anatomical structures can be elusive, with the combined translational and rotational motions of the knee joint. Moreover, these interrelationships may change over the course of the surgery as structures are removed and/or reconstructed.
One method for determining the femoral point of graft attachment during ACL replacement is disclosed in European Patent Application No. 0603089 to Cinquin et al., which is hereby incorporated by reference in its entirety. The disclosed method concerns the determination of a femoral point of graft attachment with respect to a tibial graft attachment point such that the distance between these two points remains invariant during knee flexion and extension. The positions within an on-site three-dimensional coordinate system of a reference and a pointer, which are both provided with energy emitting markers, are determined by means of a three-dimensional position measurement system, such as the OPTOTRAK position measurement system, Northern Digital, Waterloo, On. The position measurement system measures the position of the markers with respect to the on-site three-dimensional coordinate system. Therewith, the position of the tip of the pointer is determinable by means of a computer.
The Cinquin method includes the steps of (1) attachment of a first reference at the tibia; (2) positioning of the pointer tip at a previously determined point T1 and measuring the position of the pointer tip with respect to the first reference; (3) positioning of the pointer tip at several points P1 at the trochlea of the femur close to that position where the invariant point is expected; (4) calculation of the distances of point T1 and each of the points P1; (5) displacement of the femur with respect to the tibia and calculation of the variations of the distances between T1 and each of the points P1; and (6) selection of that point P1 among points P1 which shows the most invariant distance.
The Cinquin method assumes that the optimal placement of a graft is determined by a simple isometric elongation criterion which relies on the ligament trajectory following the course of a straight line (i.e. the trajectory is not dependent on the intrusion of the bone surfaces nor the ligament thickness).
Ligament grafts in reality are complex structures that have an appreciable thickness (e.g., 8-10 mm in diameter), which can affect their function and geometry during joint motion. In particular, the course of these ligament bundles is often guided by the curved protruding surfaces on the ends of the femur and on the tibia in the vicinity of the ligament attachment sites. Indeed, the collateral and posterior structures of the knee are known to wrap around the curved bone surfaces of the femur and tibia which can influence the ligament trajectory and elongation patterns during knee motion. As a result, a linear based determination of the isometric elongation characteristics of the ligament is too simplistic and does not account for the realities of a typical operative site where the ligament does not follow a purely linear path.
Another disadvantage of the Cinquin method is that only a normal flexion extension motion of the knee joint kinematics is considered in the determination of the fixation point of the ligament graft, and no consideration is taken for the surrounding structures and their associated laxities.
U.S. Pat. No. 6,725,082 (which is hereby incorporated by reference in its entirety) discloses an image-based system and method for computer-assisted ACL replacement in which landmark points are identified on the bones and on medical images of the knee (such as X rays, CT or MRI scans), the images are then registered to the patient, and a drilling tool is then navigated with respect to some “anatomical” graft placement criterion as determined on the medical images. This permits visualisation of the graft fixation points on the bones in the X-ray images, and in relation to the landmark points identified on the images. The disadvantage of this system is that no information or measurements regarding knee motion or laxity is provided. Furthermore, no means are provided for simulating realistic ligament trajectories based on the 3D shapes of the bones and of the ligament geometry such as the graft thickness and length.
Yet another disadvantage of conventional computer positioning systems, such as those described above, is that these systems are based almost entirely on determining optimal isometric coordinate points or locations for determining a fixation point on the femur and a fixation point on the tibia. However, there are a number of other considerations that should or can be taken into account when determining the optimal location of the ligament and for assisting the physician in selecting the appropriate procedure that is to be performed on the patient. For example, knee laxity data can assist the physician in determining the type of procedure to be undertaken; however, this type of data is not utilized in the conventional computer systems, which again, merely utilize isometric data to model a straight line (linear) layout for the ligament.